Nitrogen-doped carbon aerogels for electrical energy storage

ABSTRACT

Disclosed here is a composition comprising a nitrogen-doped carbon aerogel, wherein the nitrogen-doped carbon aerogel comprises a polymerization product of formaldehyde and at least one nitrogen-containing resorcinol analog. Also disclosed is a supercapacitor comprising the nitrogen-doped carbon aerogel.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/820,397 filed Aug. 6, 2015, which is hereby incorporated by referencein its entirety.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with government support under Contract No.DE-AC52-07NA27344 awarded by the U.S. Department of Energy. Thegovernment has certain rights in this invention.

BACKGROUND

Energy storage devices are becoming more important with the increasinggeneration of energy from renewable sources. Supercapacitors, also knownas electrochemical double-layer capacitors, are devices that storeelectrical energy via polarization of an electrode/electrolyteinterface. The operating mechanisms for supercapacitors include electricdouble-layer capacitance (EDLCs) and pseudocapacitance. Supercapacitorshave a higher power density and a longer lifetime than batteries, butoften suffer from a lower energy density.

Carbon aerogels have received considerable attention for energy relatedapplications such as electrode materials for supercapacitors due totheir unique combination of properties, including tunable morphology,high surface area, electrical conductivity, chemical inertness andenvironmental compatibility. Recent studies have shown that, through theincorporation of nitrogen into the hexagonal carbon lattice, nitrogendoping of some carbon nanomaterials can increase their energy density byincreasing their specific capacitance. Currently, nitrogen-doped carbonnanomaterials are mainly produced in solid state by high temperatureannealing in the presence of ammonia. Such method, however, suffers fromhigh energy consumption and low nitrogen incorporation.

Thus, a need exists for the development of a more energy efficientmethod for synthesizing nitrogen-doped carbon aerogels with a higher andcontrollable nitrogen content.

SUMMARY

One aspect of some embodiments of the invention described herein relatesto a method for making a nitrogen-doped carbon aerogel, comprising:preparing a reaction mixture comprising formaldehyde, at least onenitrogen-containing resorcinol analog, at least one catalyst, and atleast one solvent; curing the reaction mixture to produce a wet gel;drying the wet gel to produce a dry gel; and thermally annealing the drygel to produce the nitrogen-doped carbon aerogel.

Another aspect of some embodiments of the invention described hereinrelates to a composition comprising a nitrogen-doped carbon aerogelobtained according to the disclosed method.

A further aspect of some embodiments of the invention described hereinrelates to a supercapacitor comprising the nitrogen-doped carbonaerogel.

These and other features, together with the organization and manner ofoperation thereof, will become apparent from the following detaileddescription when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Example nitrogen functionalities that can be incorporated into acarbon lattice: (a) pyrrole, (b) primary amine, (c) secondary amine, (d)pyridine, (e) imine, (f) tertiary amine, (g) nitro, (h) nitroso, (i)amide, (j) pyridine, (k) pyridine-N-oxide and (l) quaternary nitrogen.

FIG. 2: Cyclic voltammograms for the aminophenol, pyridinediol, andresorcinol gels in 1M KOH with a scan rate of 6 mV/sec. When normalizedby surface area, the N-doped samples demonstrate higher capacitance dueto enhanced quantum capacitance and/or pseudocapacitive contributions.

FIG. 3: Ragone plot (gravimetric) shows better performance (higher powerand energy) for N-doped (pyridinediol derived) samples compared withresorcinol control sample, despite lower specific capacitance.

DETAILED DESCRIPTION

Reference will now be made in detail to some specific embodiments of theinvention contemplated by the inventors for carrying out the invention.Certain examples of these specific embodiments are illustrated in theaccompanying drawings. While the invention is described in conjunctionwith these specific embodiments, it will be understood that it is notintended to limit the invention to the described embodiments. On thecontrary, it is intended to cover alternatives, modifications, andequivalents as may be included within the spirit and scope of theinvention as defined by the appended claims.

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention.Particular example embodiments of the present invention may beimplemented without some or all of these specific details. In otherinstances, well known process operations have not been described indetail in order not to unnecessarily obscure the present invention.

Various techniques and mechanisms of the present invention willsometimes be described in singular form for clarity. However, it shouldbe noted that some embodiments include multiple iterations of atechnique or multiple instantiations of a mechanism unless notedotherwise.

Methods for Making Nitrogen-Doped Carbon Aerogels

Nitrogen-doped carbon aerogels can be synthesized by a polymer approachsimilar to the synthesis of carbon aerogels, albeit using at least oneresorcinol analog (e.g., pyridinediol and aminophenol) as thenitrogen-containing precursor. Using this approach, the amount ofnitrogen in the material can be controlled by synthesizing polymer gelscontaining different ratios of the resorcinol analog (e.g., pyridinedioland aminophenol), varying the pyrolysis temperature, and/or or changingthe solvent and catalyst used in the synthesis.

Many embodiments of the invention described herein relates to a methodfor making a nitrogen-doped carbon aerogel, comprising: preparing areaction mixture comprising formaldehyde, at least onenitrogen-containing resorcinol analog, at least one catalyst, and atleast one solvent; curing the reaction mixture to produce a wet gel;drying the wet gel to produce a dry gel; and thermally annealing the drygel to produce the nitrogen-doped carbon aerogel. Although the use offormaldehyde is preferred, the present invention also encompasses theuse of another aldehyde or another carbonyl-containing compound in placeof, or in combination with, formaldehyde.

The nitrogen-containing resorcinol analog includes nitrogen-containingcompounds that are structurally similar to resorcinol and capable ofpolymerizing with formaldehyde in a sol-gel reaction. In someembodiments, the nitrogen-containing resorcinol analog comprises anaromatic ring linked to at least one hydroxyl group and optionally atleast one amine, amide, nitro, nitroso, or imine group, wherein saidaromatic ring optionally comprises at least one nitrogen atom in thering. Said aromatic ring can be, for example, a five- or six-memberedring selected from the group consisting of benzene, pyridine, pyrazine,pyrimidine, pyridazine, pyrrole, pyrazole and imidazole. In someembodiments, the nitrogen-containing resorcinol analog does not comprisea triazine ring.

In some embodiments, the nitrogen-containing resorcinol analog comprisesa nitrogen-containing ring linked to at least two hydroxyl groups,wherein the nitrogen-containing ring is selected from the groupconsisting of pyridine, pyrazine, pyrimidine, pyridazine, pyrrole,pyrazole and imidazole, wherein the nitrogen-containing ring can beoptionally further derivatized.

In some embodiments, the nitrogen-containing resorcinol analog comprisesa benzene ring or a nitrogen-containing ring linked to at least onehydroxyl group and at least one amine group, wherein thenitrogen-containing ring is selected from the group consisting ofpyridine, pyrazine, pyrimidine, pyridazine, pyrrole, pyrazole andimidazole, wherein the benzene ring or the nitrogen-containing ring canbe optionally further derivatized.

In some embodiments, the nitrogen-containing resorcinol analog ispyridinediol, such as 2,3-pyridinediol, 2,4-pyridinediol,2,5-pyridinediol, 3,4-pyridinediol, and 3,5-pyridinediol. In someembodiments, the nitrogen-containing resorcinol analog is aminophenol,such as 2-aminophenol, 3-aminophenol, and 4-aminophenol.

The reaction mixture comprises at least one solvent. In someembodiments, the solvent is water. In some embodiments, the solvent isan organic solvent. In some embodiments, the reaction mixture compriseswater and at least one organic solvent. In some embodiments, thereaction mixture comprises dimethylformamide. Other suitable organicsolvents include, but are not limited to, for example, alcohol,tetrahydrofuran, ethylene glycol, N-methylpyrrolidone. DMSO, carbonates,acetone, etc.

The reaction mixture comprises at least one catalyst. In someembodiments, the catalyst is an acid catalyst. In some embodiments, thecatalyst is a base catalyst. In some embodiments, the catalyst is aceticacid. Other suitable catalysts include, but are not limited to, forexample, nitric acid, ascorbic acid, hydrochloric acid, sulfuric acid,sodium carbonate, sodium hydroxide, ammonium hydroxide and calciumsulfate.

In some embodiments, in the reaction mixture the molar ratio of thenitrogen-containing resorcinol analog to the catalyst ranges from about1:1 to about 5000:1, or from about 5:1 to about 2000:1, or from about10:1 to about 1000:1, or from about 20:1 to about 500:1, or from about50:1 to about 200:1.

In some embodiments, in the reaction mixture the molar ratio of thenitrogen-containing resorcinol analog to the formaldehyde ranges fromabout 2:1 to about 1:10, or from about 1:1 to about 1:5, or at about1:2.

In some embodiments, the reaction mixture consists essentially of orconsists of formaldehyde, the at least one nitrogen-containingresorcinol analog, the at least one catalyst, and the at least onesolvent.

In some embodiments, the reaction mixture is substantially or totallyfree of melamine or melamine derivatives.

In some embodiments, the reaction mixture is cured at a temperature ofabout 25° C. to about 100° C. to produce a wet gel. In some embodiments,the reaction mixture is cured at a temperature of about 85° C. Thecuring time can be, for example, about 4-168 hours, or about 8-120hours, or about 12-72 hours. In some embodiments, the reaction mixtureis cured at atmospheric pressure.

In some embodiments, the wet gel is subjected to solvent exchange toremove reaction by-products. Suitable solvents include, but are notlimited to, water and acetone.

In some embodiments, the wet gel is dried under supercritical condition(e.g., using supercritical CO₂). In some embodiments, the wet gel isdried under ambient temperature and pressure. In some embodiments, thewet gel is freeze dried.

In some embodiments, the dry gel is pyrolyzed in an inert gas to producea graphene aerogel. Suitable inert gases include, but are not limitedto, for example, N₂ and noble gas. The drying temperature can be, forexample, at least about 500° C., or at least about 600° C., or at leastabout 800° C., or at least about 1000° C., or from about 500° C. toabout 1500° C., or from about 600° C. to about 1200° C., or at about1050° C.

In some embodiments, the method comprises further functionalizing thenitrogen-doped carbon aerogel with additional nitrogen-containingfunctionalities.

In one specific embodiment, formaldehyde was added as 37% aqueoussolution in a molar ratio of 1:2, nitrogen-containing precursor toformaldehyde. Acetic acid was used as a catalyst for the polymerizationreaction and was added in a molar ratio of 50:1, nitrogen-containingprecursor to acetic acid. Gel formulations were made with anitrogen-containing precursor concentration of 0.9 M indimethylformamide (DMF). The reaction mixture was then placed in a moldand cured at 85° C. for 3 d. The resulting gels were then washed inwater (3×) and then in acetone (3×) before supercritically drying withCO₂. The resulting nitrogen-containing polymer foams were pyrolyzed attemperatures ranging from 600° C. to 1200° C. to generate nitrogen-dopedcarbon aerogels.

Nitrogen-Doped Carbon Aerogels

Many embodiments of the invention relates to a composition comprising anitrogen-doped carbon aerogel obtained according to the method describedherein. Electrochemical testing of these materials by cyclic voltammetryshowed that these nitrogen-doped carbon aerogels can have a higherelectrical energy storage capacity than undoped carbon aerogels.Accordingly, these nitrogen-doped carbon aerogels can be used insupercapacitor electrodes.

In some embodiments, the nitrogen-doped carbon aerogel has a nitrogencontent of about 1 wt. % or more, or about 2 wt. % or more, or about 3wt. % or more, or about 4 wt. % or more, or about 5 wt. % or more, orabout 6 wt. % or more, or about 7 wt. % or more, or about 8 wt. % ormore, or about 9 wt. % or more, or up to about 10%.

In some embodiments, the nitrogen-doped carbon aerogel has a surfacearea of about 100 m²/g or more, or about 200 m²/g or more, or about 300m²/g or more, or about 400 m²/g or more, or about 500 m²/g or more.

In some embodiments, the nitrogen-doped carbon aerogel has a density ofabout 0.5 g/ml or less, or about 0.4 g/ml or less, or about 0.3 g/ml orless, or about 0.2 g/ml or less, or about 0.1 g/ml or less.

In some embodiments, the nitrogen-doped carbon aerogel has a specificcapacitance of about 50 F/g or more, or about 60 F/g or more, or about70 F/g or more, or about 80 F/g or more, or about 90 F/g or more, orabout 100 F/g or more.

In some embodiments, the nitrogen-doped carbon aerogel has an arealcapacitance of about 15 μF/cm² or more, or about 20 μF/cm² or more, orabout 25 μF/cm² or more, or about 30 μF/cm² or more.

In some embodiments, the nitrogen-doped carbon aerogel is in the form ofa monolith having at least one lateral dimension of 100 microns or more,or 1 mm or more, or 10 mm or more, or 100 mm or more, or 1 cm or more.

In some embodiments, the nitrogen-doped carbon aerogel comprises,consists essentially of, or consists of the polymerization product offormaldehyde and the at least one nitrogen-containing resorcinol analog.In some embodiments, said polymerization product does not comprise anytriazine rings.

Additional embodiments of the invention relates to a device comprisingthe nitrogen-doped carbon aerogel described herein. In some embodiments,the device is a supercapacitor.

Applications

The nitrogen-doped carbon aerogel described herein can be used in avariety of applications. For example, they can be used in electricalenergy storage, supercapacitors and battery electrodes, hybridcapacitors, pseudocapacitors, ultracapacitors, microbatteries, Li-ionbatteries, next-generation batteries, hybrid and electrical vehicles,small portable devices, cordless tools, airplane emergency doors, andrenewable energy applications.

WORKING EXAMPLES Example 1

Material Synthesis.

Three commercially available resorcinol analogues were investigated,pyridinediol, aminophenol, and 3,5-dihydroxyanaline. The standardresorcinol reaction was run in parallel to all tests for comparison. The3,5-dihydroxyanaline proved to be brittle after pyrolysis and wastherefore dropped from the investigation. Besides pyridinediol andaminophenol, other nitrogen-containing resorcinol analogs can also beused for making the N-doped carbon aerogel.

Resorcinol and 3-aminophenol were purchased from Sigma Aldrich.3,5-pyridinediol was purchased from Parkway Scientific. Gel formulationswere made with a substrate concentration of 0.9 M in dimethylformamide(DMF). Formaldehyde was added as 37% aqueous solution in asubstrate-to-formaldehyde molar ratio of 1:2. Acetic acid was used as acatalyst for the polymerization reaction and was added in asubstrate-to-acetic acid ratio of 50:1. The reaction mixture was thenplaced in a mold to control material shape, namely in a glass vial toform a monolith or in a thin glass mold to form a disk. The resultinggels were washed three times for twelve hours in water and then anotherthree times for twelve hours in acetone. The gels were supercriticallydried in CO₂. Finally, the gels were pyrolyzed at 1050° C.

The materials used in this study were 12% solids by mass in solution.The ratio of [reactant monomer]/[catalyst] (R/C) can affect density,surface area, and mechanical properties.

Characterization.

The as-synthesized gels were characterized by a variety of methods.Scanning Electron Microscopy (SEM) was used to visually examine themorphology of the resulting gels along with EDX to qualitatively assessatomic composition. The BET surface area was determined. Cyclicvoltammetry was used to measure the electrochemical properties. X-rayphotoelectron spectroscopy (XPS) was used to quantitatively determinenitrogen content as well as the type of nitrogen species incorporatedinto the materials (see FIG. 1 for examples of nitrogen species that canbe incorporated into nitrogen-doped carbon aerogels).

Results.

The properties of N-doped carbon aerogels produced from pyridinediol,aminophenol, and resorcinol are summarized in Table 1. The specificcapacitance of N-containing materials is lower than the resorcinolcontrol sample. However, when the capacitance is normalized by surfacearea, the capacitance of the N-containing materials is larger than thatof the control sample due to quantum capacitance (changes to theelectronic structure of the carbon lattice) and/or pseudocapacitive(fast redox/donor-acceptor) contributions (see FIG. 2).

TABLE 1 Summary of properties of materials produced from pyridinediol,aminophenol, and resorcinol. Specific Areal Density Suface AreaCapacitance Capacitance Starting Material (g/mL) (m²/g) (F/g) (μF/cm²)Pyridinediol 0.185 383 90 23.5 Aminophenol 0.297 261 78 29.4 Resorcinol0.329 497 114 22.9

Two electrode cells were assembled to mimic a real-world charge-storagedevice. The Ragone plots in FIG. 3 illustrate that thepyridinediol-derived material has superior specific energy and powerthan the resorcinol control sample despite its lower specificcapacitance.

As used herein, the singular terms “a,” “an,” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to a compound can include multiple compounds unlessthe context clearly dictates otherwise.

As used herein, the terms “substantially,” “substantial,” and “about”are used to describe and account for small variations. When used inconjunction with an event or circumstance, the terms can refer toinstances in which the event or circumstance occurs precisely as well asinstances in which the event or circumstance occurs to a closeapproximation. For example, the terms can refer to less than or equal to±10%, such as less than or equal to ±5%, less than or equal to ±4%, lessthan or equal to ±3%, less than or equal to ±2%, less than or equal to±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or lessthan or equal to ±0.05%.

Additionally, amounts, ratios, and other numerical values are sometimespresented herein in a range format. It is to be understood that suchrange format is used for convenience and brevity and should beunderstood flexibly to include numerical values explicitly specified aslimits of a range, but also to include all individual numerical valuesor sub-ranges encompassed within that range as if each numerical valueand sub-range is explicitly specified. For example, a ratio in the rangeof about 1 to about 200 should be understood to include the explicitlyrecited limits of about 1 and about 200, but also to include individualratios such as about 2, about 3, and about 4, and sub-ranges such asabout 10 to about 50, about 20 to about 100, and so forth.

In the foregoing description, it will be readily apparent to one skilledin the art that varying substitutions and modifications may be made tothe invention disclosed herein without departing from the scope andspirit of the invention. The invention illustratively described hereinsuitably may be practiced in the absence of any element or elements,limitation or limitations, which is not specifically disclosed herein.The terms and expressions which have been employed are used as terms ofdescription and not of limitation, and there is no intention that in theuse of such terms and expressions of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention. Thus, it should be understood that although the presentinvention has been illustrated by specific embodiments and optionalfeatures, modification and/or variation of the concepts herein disclosedmay be resorted to by those skilled in the art, and that suchmodifications and variations are considered to be within the scopes ofthis invention.

What is claimed is:
 1. A composition comprising a nitrogen-doped carbon aerogel, wherein the nitrogen-doped carbon aerogel consists essentially of polymerization product of formaldehyde and at least one nitrogen-containing resorcinol analog.
 2. The composition of claim 1, wherein the resorcinol analog is selected from (a) an aromatic compound comprising a nitrogen-containing ring linked to at least two hydroxyl groups and (b) an aromatic compound comprising a benzene ring or nitrogen-containing ring linked to at least one hydroxyl group and at least one amine group, wherein the nitrogen-containing ring is selected from the group consisting of pyridine, pyrazine, pyrimidine, pyridazine, pyrrole, pyrazole and imidazole.
 3. The composition of claim 1, wherein the resorcinol analog comprises pyridinediol.
 4. The composition of claim 3, wherein the pyridinediol comprises 2,3-pyridinediol, 2,4-pyridinediol, 2,5-pyridinediol, 3,4-pyridinediol, and/or 3,5-pyridinediol.
 5. The composition of claim 1, wherein the resorcinol analog comprises aminophenol.
 6. The composition of claim 5, wherein the aminophenol comprises 2-aminophenol, 3-aminophenol, and/or 4-aminophenol.
 7. The composition of claim 1, wherein the nitrogen-doped carbon aerogel consists of the polymerization product of formaldehyde and the at least one nitrogen-containing resorcinol analog.
 8. The composition of claim 1, wherein the nitrogen-doped carbon aerogel has a nitrogen content of about 4 wt. % or more.
 9. The composition of claim 1, wherein the nitrogen-doped carbon aerogel has a nitrogen content of about 8 wt. % or more.
 10. The composition of claim 1, wherein the nitrogen-doped carbon aerogel has a surface area of about 100 m²/g or more.
 11. The composition of claim 1, wherein the nitrogen-doped carbon aerogel has a surface area of about 300 m²/g or more.
 12. The composition of claim 1, wherein the nitrogen-doped carbon aerogel has a density of about 0.5 g/ml or less.
 13. The composition of claim 1, wherein the nitrogen-doped carbon aerogel has a density of about 0.3 g/ml or less.
 14. The composition of claim 1, wherein the nitrogen-doped carbon aerogel has a specific capacitance of about 50 F/g or more.
 15. The composition of claim 1, wherein the nitrogen-doped carbon aerogel has a specific capacitance of about 80 F/g or more.
 16. The composition of claim 1, wherein the nitrogen-doped carbon aerogel has an areal capacitance of about 20 μF/cm² or more.
 17. The composition of claim 1, wherein the nitrogen-doped carbon aerogel has an areal capacitance of about 25 μF/cm² or more.
 18. The composition of claim 1, wherein the nitrogen-doped carbon aerogel is in a form of a monolith having at least one dimension of 100 microns or more.
 19. A supercapacitor comprising the composition of claim
 1. 